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Analog Electronics(Course Code: EE314)
Lecture 35: Frequency Response contd..Feedback
Indian Institute of Technology Jodhpur, Year 2018
Feedback
Course Instructor: Shree PrakashTiwari
Email: [email protected]
b h //h / /Webpage: http://home.iitj.ac.in/~sptiwari/
Course related documents will be uploaded on http://home.iitj.ac.in/~sptiwari/EE314/
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Note: The information provided in the slides are taken form text books for microelectronics (including Sedra & Smith, B. Razavi), and various other resources from internet, for teaching/academic use only
Impedance of Parallel RC Circuit
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Small‐Signal Model for CE Stage
… Applying Miller’s Theorem
CRgCR CminThevinp
1
1,
1
C
RgCR
CmoutC
outp1
1
1,
Note that p,out > p,in
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CE Stage Pole Frequencies, for VA<∞
CrRgCR oCminThevinp
1
1,
1
Note that p,out > p,in
CrRg
CrRoCm
outoC
outp1
1
1,
I/O Impedances of CE Stage
1 1
r
CrRgCjZ
oCmin ||
1
1
oC
CSout rR
CCjZ ||||
1
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Small‐Signal Model for CS Stage
0
… Applying Miller’s Theorem
GDDminThevinp CRgCR
1
1,
1
GDDm
outD
outp
CRg
CR1
1
1,
Note that p,out > p,in
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I/O Impedances of CS Stage
0
GDDmGSin CRgCj
Z
1
1
DDBGD
out RCCj
Z ||1
• Note that there is no capacitance between input & output nodes
No Miller multiplication effect!
CB Stage: Pole Frequencies
1
YCYp CR
1,
CSY CCC
or
CB stage with BJT capacitances shown
T
Xm
S
Xp
Cg
R
1||
1,
CCX
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CG Stage: Pole Frequencies
X
1
CG stage with MOSFET capacitances shown
Xm
S
Xp
Cg
R
1||
,
SBGSX CCC
1
0
YDYp CR
1,
DBGDY CCC
Emitter Follower
• Recall that the emitter follower provides high input impedance and low output impedance, and is used as a voltage buffer.
Follower stage with BJT capacitances shown• CL is the load capacitance
Circuit for small-signal analysis (Av)
or
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AC Analysis of Emitter Follower
• KCL at node X:
vvv outX
• KCL at output node:
011
Cj
v
r
v
Cj
vv
R
vvv out
S
inout
L
outm
Cj
vvg
Cj
v
r
v
11
1)()(
)(1
2
jbja
jg
C
v
v m
in
out
m
LS
mS
LLm
S
g
C
r
R
g
CCRb
CCCCCCg
Ra
1
Ljj
Follower: Zero and Pole Frequencies
1)()(
)(1
2
jbj
jg
C
v mout
LS
LLm
S
CRC
CCCCCCg
Ra
• The follower has one zero:
• The follower has two poles at lower frequencies:
1)()( 2 jbjavinm
LS
mS g
C
r
R
g
CCRb
1
Tm
z fC
g
2
The follower has two poles at lower frequencies:
21
2 1 11)()(pp
jjjbja
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Emitter Follower: Input Capacitance
• Recall that the voltage gain of an emitter follower is
Follower stage with BJT capacitances shown mL
Lv
gR
RA
1
or
Lm
vX Rg
CCAC
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• CXY can be decomposed into CX and CY at the input and output nodes, respectively:
Lmin Rg
CCC
1
LmvY Rg
CC
AC
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Lin RrR 1
Emitter Follower: Output Impedance
Circuit for small-signal analysis (Rout)or
Cjrvgiv mX
1
Crj
CrRRrj
Rr
jCr
RrjCrR
i
vZ SSSSS
X
Xout
/11
/1
11
Smxx Rvgivv
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Emitter Follower as Active Inductor
Cj
CrRRrj
Rr
jCr
RrjCrR
i
vZ SSSSS
X
Xout
/11
/1
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CASE 1: RS < 1/gm CASE 2: RS > 1/gm
Cr/1
capacitive behavior inductive behavior
• A follower is typically used to lower the driving impedance
RS > 1/gm so that the “active inductor” characteristic on the right is usually observed.
Cascode Stage
• Review:– A CE stage has large Rin but suffers from the Miller effect.
– A CB stage is free from the Miller effect, but has small Rin.
• A cascode stage provides high Rin with minimal Miller effect.
11
21,
mm
Y
XXYv g
gv
vAor
XYX CC 2
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Cascode Stage: Pole Frequencies
, 2||
1CCrRXp
Cascode stage with BJT capacitances shown
(Miller approximation applied)
111 2|| CCrRS
1212
,
21
1
CCC
g CSm
Yp
22 2 T
mY f
g Note that
or
22,
1
CCR CSL
outp
22
, 2 TYp fC
Note that
Cascode Stage: I/O Impedances
or
111 2
1||
CCjrZin
22
1||
CSLout CCj
RZ
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Summary of Cascode Stage Benefits
• A cascode stage has high output impedance, which is advantageous for
hi i hi h l i– achieving high voltage gain
– use as a current source
• In a cascode stage, the Miller effect is reduced, for improved performance at high frequencies.
MOS Cascode Stage
• For a cascode stage, Miller multiplication is smaller than in the CS stage.
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21,
mm
Y
XXYv g
gv
vA
XYX CC 2 XYX CC 2
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Cascode Stage: Pole Frequencies
1
Cascode stage with MOSFET capacitances shown
(Miller approximation applied)0
1
22,
GDDBDoutp CCR
12
11
,
1
1
GDm
mGSG
Xp
Cgg
CR
211
221
2
,
11
1
SBGDm
mGSDB
m
Yp
CCgg
CCg
Cascode Stage: I/O Impedances
0
12
11 1
1
GDm
mGS
in
Cgg
Cj
Z
22
1||
DBGDDout CCj
RZ
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Next
• Feedback
Negative Feedback System
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• A negative feedback system consists of four components: 1) feedforward system, 2) sense mechanism, 3) feedback network, and 4) comparison mechanism.
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Close‐loop Transfer Function
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1
1
1 KA
A
X
Y
Feedback Example
121
2
1
1 ARR
RA
X
Y
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• A1 is the feedforward network, R1 and R2 provide the sensing and feedback capabilities, and comparison is provided by differential input of A1.
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Comparison Error
E
KA
XE
11
29
• As A1K increases, the error between the input and fed back signal decreases. Or the fed back signal approaches a good replica of the input.
Comparison Error
11R
R
X
Y
30
2RX
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Loop Gain
test
N
V
VKA 1
0X
31
• When the input is grounded, and the loop is broken at an arbitrary location, the loop gain is measured to be KA1.
Example: Alternative Loop Gain Measurement
32
testN VKAV 1
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Incorrect Calculation of Loop Gain
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• Signal naturally flows from the input to the output of a feedforward/feedback system. If we apply the input the other way around, the “output” signal we get is not a result of the loop gain, but due to poor isolation.
Gain Desensitization
1KAY 1
34
• A large loop gain is needed to create a precise gain, one that does not depend on A1, which can vary by ±20%.
11 KAKX
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Ratio of Resistors
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• When two resistors are composed of the same unit resistor, their ratio is very accurate. Since when they vary, they will vary together and maintain a constant ratio.
Merits of Negative Feedback
1) B d id h• 1) Bandwidth enhancement
• 2) Modification of I/O Impedances
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• 3) Linearization
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Next
• Feedback contd…